1. Field of the Invention
The invention relates to a premixture composition for the production of water-resistant photographic support material coated with polyolefin resin.
2. Brief Description of the Background of the Invention Including Prior Art
Water-resistant photographic support materials comprise a plastic film or a base paper with synthetic resin coatings applied to preferably both sides, which include polymers, for example polyethylene, and which are applied by way of extrusion coating or which comprise an organic lacquer mixture, which is placed on the paper by way of a submersion or spray method and which is dried and hardened or solidified by employing heat or energy-rich radiation.
One or several light-sensitive layers containing silver halides are applied on one of the resin layers. In case of light-sensitive layers, reference is made to both black and white as well as color photographic layers.
The synthetic resin layer (front-side coating) disposed under the light-sensitive layer or, respectively, light-sensitive layers comprises usually a light-reflecting white pigment as well as possibly color pigments, optical brighteners and/or other additives such as antistatic agents, dispersion additives for the white pigment, antioxidants, separating agents, and the like.
The synthetic resin layer, disposed on the paper side opposite to that of the one with the light-sensitive layers (back-side coating), can be pigmented or can be left unpigmented and/or can contain other additives, which result from the use of the laminate as a photographic support material in each case and which, in principle, can correspond to those of the front-side coating.
The front-side coating can contain additional function layers which improve, for example, the adhesion of the light-sensitive layers.
In addition, the back-side coating can be provided with additional function layers which can improve, for example, the capability of writing on the material, the antistatic property, the slip properties, the flatness (planarity), or several of these properties.
In order to achieve that a resin coated photographic paper support, after a one-side application of photographic layers, does not exhibit an undesirable curvature (curl), it is usually provided that the front-side polyethylene layer comprises substantially a low-density polyethylene (LDPE), whereas the back-side polyethylene layer predominantly comprises a high-density polyethylene (HDPE), compare German Patent Application Laid Open DE-OS 2,028,600.
It is a joint property of the two polyethylene types and of their mixtures that they can only be filled to a limited extent with pigments. It is taught in German Patent Application Laid Open DE-OS 3,411,681, having an English-language equivalent=GB 2,138,964, how much the sharpness of a photographic image depends on the extent of reflection of the impinging light off the white pigments. Therefore, it is an important goal of all light-sensitive support materials to improve this reflection of the impinging light. This is achieved by employing white pigments with the highest indices of refraction, such as titanium dioxide, and by maintaining as high as possible the content in white pigment in the polyolefin resin with a very good dispersion such that a dense pigment packing is generated and present in the support near the surface. It has to be avoided in this context to generate pigment agglomerates in the polyolefin resin, since they decrease on the one hand the total light reflection and, on the other hand, they can result in disturbances and interferences during the drawing of the melt film in the extrusion coating, or later during the casting of the resin coated support with light-sensitive emulsions.
In practice of extrusion coating, the pigment concentration in a polyethylene resin coating mixture has been limited hitherto to less than 20 weight-percent and is usually between 10 and 17 weight-percent. This holds both for a pure low-density polyethylene as well as for the described mixtures of low-density polyethylene and high-density polyethylene.
For the pigmentation of polyolefin coatings, pigments are therefore not applied as such, but rather in form of premixture composition, the so-called master batch, which contains a desired pigment already dispersed in a resin to a certain predetermined concentration. The master batch is then diluted with diluting resins to prepare a resin composition with a desired content of titanium dioxide.
As for the resin suitable for the master batch, low-density-polyethylene resin (LDPE) is conventionally used, as taught, for example, in U.S. Pat. No. 4,650,747.
The production of such master batches takes place in a special operational process. Typically suited for this purpose are kneading machines, roller mills and extruders.
According to the presently existing state of the art, various methods for the production of master batches have been proposed, for example in U.S. Pat. No. 4,650,747. These methods, however, do not present a satisfactory and/or solution in every desirable respect and the distribution of the pigment particles leaves room for improvement.